Blender export as dae file contains broken 3D render
To me it looks like the face normals are facing different ways. Game engines use backface culling to only draw one side of a polygon.

In edit mode select all A and use &#9096; CtrlN or &#9096; Ctrl&#8679; ShiftN to set the normals to the same direction.

You may also want to look at not using backface culling so that both sides of the robe are visible in your game.